A calculation method for Z-factor of natural gas based on BWRS equation

Z-factor of natural gas is a necessary parameter in the calculation of reservoir engineering, and plays an important role in many engineering applications of oil and gas exploration and development. The exponential term in BWRS equation was modified based on the BWR equation modified by Starling and the DAK method. Then, a new Z-factor calculation method was developed by means of non-linear regression analysis. Finally, the DAK method, the DAK method modified by HU Jianguo and the new method were compared by using the standard data of Z-factor. The error analysis results indicate that in the situations with general temperature and pressure range (1.05≤T_pr≤3.0 & 0.2≤p_pr≤15) and higher pressure (1.4≤T_pr≤2.8 & 15≤p_pr≤30) (7 148 Z-factors of natural gas), the average absolute error of the new method is 0.382% and 0.205%, respectively, which are higher than the calculation accuracy of the DAK method and the DAK method modified by HU Jianguo. Comparatively, DAK method has higher calculation accuracy when pressure and temperature is in range of 1.1≤T_pr≤3.0 & 0.2≤p_pr≤15, the DAK method modified by HU Jianguo is only applicable to higher pressure(1.4≤T_pr≤2.8 & 15≤p_pr≤30), and the new method has wider applicable range (1.05 < T_pr≤3.0 & 0.2≤p_pr≤15以及1.4≤T_pr≤2.8 & 15≤p_pr≤30) with higher calculation accuracy.